(1) Field of the Invention
This invention relates to a process for purifying a gaseous stream, and more particularly to a one-step process for removing parts per million levels of impurities from an inert gaseous stream.
(2) Description of the Prior Art
As the semiconductor industry is developing integrated circuits with ever increasing line densities, the manufacturing processes employed require that materials utilized are as free of impurities as is possible. Inert gas, such as nitrogen or argon, is frequently utilized in semiconductor manufacturing processes and while commercially available nitrogen and argon are relatively pure, it is necessary to assure that purities are maintained so as to avoid contamination of semiconductor materials by impurities, such as H.sub.2, H.sub.2 O, CO, CO.sub.2, O.sub.2, and the like.
Although nitrogen will react with certain elements under particular conditions, it will be understood that the term "inert gas" as used herein includes nitrogen. It has previously been proposed to remove certain of the above mentioned impurities, e.g., O.sub.2 by catalytically combining oxygen with hydrogen over a catalyst, such as DeOxo D; however, such process requires relatively high temperatures (e.g. 450.degree. C.) to assure catalytic combustion of the O.sub.2 to the extent necessary to form H.sub.2 O. Therefore, it is then necessary to cool the hot "purified" inert gas in a heat exchanger or like equipment prior to process usage, thereby adding a step to the overall purification process. A typical catalytic process for reacting hydrogen with oxygen is disclosed in Japanese published Patent Application No. 59-54608.
It is known to use zeolites to adsorb oxygen in inert gas streams, and typically such processes involve coling a zeolite bed to a very low temperature, e.g. below about -220.degree. F. Low temperatures or cryogenic conditions, in turn, require special equipment, materials, insulation, and the like. A typical adsorption process of this type is disclosed in U.S. Pat. No. 3,928,004. It is also known to utilize zeolites to remove CO.sub.2 from air or inert gas streams at ambient temperatures. Such a method is disclosed in U.S. Pat. No. 3,885,927. While it would appear from this reference that carbon dioxide may be removed from air or inert gas streams, it does not appear that the adsorbents, as disclosed therein, are effective to remove other impurities, such as O.sub.2, H.sub.2, or CO, nor is such capability suggested in such reference.
Other techniques for removing oxygen from gaseous streams include the use of copper-based getter materials, such as described in Japanese Patent Application No. 53-33312 wherein the getter material is heated to a temperature of at least 150.degree. C., and cooled prior to use. Such process is only effective to remove oxygen and not impurities, such as H.sub.2 O and CO.sub.2. The use of nickel-based materials to remove oxygen only is disclosed in U.S. Pat. No. 3,682,585 and in Japanese Patent No. Sho 50(1975)-6440.
In co-pending U.S. application Ser. No. 717,055, filed Mar. 28, 1985, now U.S. Pat. No. 4,579,723 there is disclosed a two-step process for removing parts per million level of gaseous impurities, i.e. carbon monoxide, carbon dioxide, hydrogen, water vapor and oxygen from an inert gas stream, such as nitrogen or argon, at substantially ambient temperatures. In a first step of the process, the gas stream is passed through a bed of catalytic material, such as platinum-rhodium on an alumina substrate, wherein carbon monoxide and hydrogen react with oxygen to form carbon dioxide and water with water being at least partially adsorbed in the catalyst. In a second stage of the process, the thus treated gaseous stream is passed through a bed of a getter material, such as copper on an alumina substrate, wherein oxygen reacts therewith and carbon dioxide and H.sub.2 O are trapped therein to produce an inert gas stream substantially free of gaseous impurities, generally less than about 1.0 part per million of such impurities. Such a two-stage process requires an elaborate piping and valving system as well as intricate fluid flow configuration to effect regeneration of the respective catalytic and getter beds.